Intravenous infusion of donor apoptotic leukocytes before transplantation delays allogeneic islet graft rejection through regulatory T cells

Apoptotic Donor Cells in Transplantation

Despite significant advances in prevention and treatment of transplant rejection with immunosuppressive medications, we continue to face challenges of long-term graft survival, detrimental medication side effects to both the recipient and transplanted organ together with risks for opportunistic infections. Transplantation tolerance has so far only been achieved through hematopoietic chimerism, which carries with it a serious and life-threatening risk of graft versus host disease, along with variability in persistence of chimerism and uncertainty of sustained tolerance. More recently, numerous in vitro and in vivo studies have explored the therapeutic potential of silent clearance of apoptotic cells which have been well known to aid in maintaining peripheral tolerance to self. Apoptotic cells from a donor not only have the ability of down regulating the immune response, but also are a way of providing donor antigens to recipient antigen-presenting-cells that can then promote donor-specific peripheral tolerance. Herein, we review both laboratory and clinical evidence that support the utility of apoptotic cell-based therapies in prevention and treatment of graft versus host disease and transplant rejection along with induction of donor-specific tolerance in solid organ transplantation. We have highlighted the potential limitations and challenges of this apoptotic donor cell-based therapy together with ongoing advancements and attempts made to overcome them.

SLE: Novel Postulates for Therapeutic Options

Genetic deficiency in C1q is a strong susceptibility factor for systemic lupus erythematosus (SLE). There are two major hypotheses that potentially explain the role of C1q in SLE. The first postulates that C1q deficiency abrogates apoptotic cell clearance, leading to persistently high loads of potentially immunogenic self-antigens that trigger autoimmune responses. While C1q undoubtedly plays an important role in apoptotic clearance, an essential biological process such as removal of self- waste is so critical for host survival that multiple ligand-receptor combinations do fortunately exist to ensure that proper disposal of apoptotic debris is accomplished even in the absence of C1q. The second hypothesis is based on the observation that locally synthesized C1q plays a critical role in regulating the earliest stages of monocyte to dendritic cell (DC) differentiation and function. Indeed, circulating C1q has been shown to keep monocytes in a pre-dendritic state by silencing key molecular players and ensuring that unwarranted DC-driven immune responses do not occur. Monocytes are also able to display macromolecular C1 on their surface, representing a novel mechanism for the recognition of circulating "danger." Translation of this danger signal in turn, provides the requisite "license" to trigger a differentiation pathway that leads to adaptive immune response. Based on this evidence, the second hypothesis proposes that deficiency in C1q dysregulates monocyte-to-DC differentiation and causes inefficient or defective maintenance of self-tolerance. The fact that C1q receptors (cC1qR and gC1qR) are also expressed on the surface of both monocytes and DCs, suggests that C1q/C1qR may regulate DC differentiation and function through specific cell-signaling pathways. While their primary ligand is C1q, C1qRs can also independently recognize a vast array of plasma proteins as well as pathogen-associated molecular ligands, indicating that these molecules may collaborate in antigen recognition and processing, and thus regulate DC-differentiation. This review will therefore focus on the role of C1q and C1qRs in SLE and explore the gC1qR/C1q axis as a potential target for therapy.

Immune Tolerance Induction Using Cell-Based Strategies in Liver Transplantation: Clinical Perspectives

Liver transplantation (LT) has become the best chance and a routine practice for patients with end-stage liver disease and small hepatocellular carcinoma. However, life-long immunosuppressive regimens could lead to many post-LT complications, including cancer recurrence, infections, dysmetabolic syndrome, and renal injury. Impeccable management of immunosuppressive regimens is indispensable to ensure the best long-term prognosis for LT recipients. This is challenging for these patients, who probably have a post-LT graft survival of more than 10 or even 20 years. Approximately 20% of patients after LT could develop spontaneous operational tolerance. They could maintain normal graft function and histology without any immunosuppressive regimens. Operational tolerance after transplantation has been an attractive and ultimate goal in transplant immunology. The liver, as an immunoregulatory organ, generates an immune hyporesponsive microenvironment under physiological conditions. In this regard, LT recipients may be ideal candidates for studies focusing on operative tolerance. Cell-based strategies are one of the most promising methods for immune tolerance induction, including chimerism induced by hematopoietic stem cells and adoptive transfer of regulatory T cells, regulatory dendritic cells, regulatory macrophages, regulatory B cells, and mesenchymal stromal cells. The safety and the efficacy of many cell products have been evaluated by prospective clinical trials. In this review, we will summarize the latest perspectives on the clinical application of cell-based strategies in LT and will address a number of concerns and future directions regarding these cell products.

Harnessing Apoptotic Cells for Transplantation Tolerance: Current Status and Future Perspectives

PURPOSE OF REVIEW

The use of donor apoptotic cells is an emerging therapy for inducing transplantation tolerance. In this review, we will discuss current understanding of mechanisms of this approach, as well as crucial aspects necessary for successful translation of this approach to clinical transplantation.

RECENT FINDINGS

Transplantation tolerance by donor apoptotic cells is mediated by their homeostatic interaction with recipient phagocytes, and subsequent expansion of suppressor cell populations as well as inhibition of effector T cells via deletion and anergy. To ensure their tolerogenicity, it is critical to procure non-stressed donor cells, and to induce and arrest their apoptosis at the appropriate stage prior to their administration. Equally important is the monitoring of dynamics of recipient immunological status, and its influences on tolerance efficacy and longevity. Emerging concepts and technologies may significantly streamline tolerogen manufacture and delivery of this approach, and smooth its transition to clinical application.

SUMMARY

Hijacking homeostatic clearance of donor apoptotic cells is a promising strategy for transplantation tolerance. Timing is now mature for concerted efforts for transitioning this strategy to clinical transplantation.

The many ways tissue phagocytes respond to dying cells

Apoptosis is an important component of normal tissue physiology, and the prompt removal of apoptotic cells is equally essential to avoid the undesirable consequences of their accumulation and disintegration. Professional phagocytes are highly specialized for engulfing apoptotic cells. The recent ability to track cells that have undergone apoptosis in situ has revealed a division of labor among the tissue resident phagocytes that sample them. Macrophages are uniquely programmed to process internalized apoptotic cell-derived fatty acids, cholesterol and nucleotides, as a reflection of their dominant role in clearing the bulk of apoptotic cells. Dendritic cells carry apoptotic cells to lymph nodes where they signal the emergence and expansion of highly suppressive regulatory CD4 T cells. A broad suppression of inflammation is executed through distinct phagocyte-specific mechanisms. A clever induction of negative regulatory nodes is notable in dendritic cells serving to simultaneously shut down multiple pathways of inflammation. Several of the genes and pathways modulated in phagocytes in response to apoptotic cells have been linked to chronic inflammatory and autoimmune diseases such as atherosclerosis, inflammatory bowel disease and systemic lupus erythematosus. Our collective understanding of old and new phagocyte functions after apoptotic cell phagocytosis demonstrates the enormity of ways to mediate immune suppression and enforce tissue homeostasis.

Peripheral blood mononuclear cell secretome for tissue repair

For almost two decades, cell-based therapies have been tested in modern regenerative medicine to either replace or regenerate human cells, tissues, or organs and restore normal function. Secreted paracrine factors are increasingly accepted to exert beneficial biological effects that promote tissue regeneration. These factors are called the cell secretome and include a variety of proteins, lipids, microRNAs, and extracellular vesicles, such as exosomes and microparticles. The stem cell secretome has most commonly been investigated in pre-clinical settings. However, a growing body of evidence indicates that other cell types, such as peripheral blood mononuclear cells (PBMCs), are capable of releasing significant amounts of biologically active paracrine factors that exert beneficial regenerative effects. The apoptotic PBMC secretome has been successfully used pre-clinically for the treatment of acute myocardial infarction, chronic heart failure, spinal cord injury, stroke, and wound healing. In this review we describe the benefits of choosing PBMCs instead of stem cells in regenerative medicine and characterize the factors released from apoptotic PBMCs. We also discuss pre-clinical studies with apoptotic cell-based therapies and regulatory issues that have to be considered when conducting clinical trials using cell secretome-based products. This should allow the reader to envision PBMC secretome-based therapies as alternatives to all other forms of cell-based therapies.

Concise Review: Apoptotic Cell-Based Therapies-Rationale, Preclinical Results and Future Clinical Developments

The objectives of this review are to summarize the experimental data obtained using apoptotic cell-based therapies, and then to discuss future clinical developments. Indeed, apoptotic cells exhibit immunomodulatory properties that are reviewed here by focusing on more recent mechanisms. These immunomodulatory mechanisms are in particular linked to the clearance of apoptotic cells (called also efferocytosis) by phagocytes, such as macrophages, and the induction of regulatory T cells. Thus, apoptotic cell-based therapies have been used to prevent or treat experimental inflammatory diseases. Based on these studies, we have identified critical steps to design future clinical trials. This includes: the administration route, the number and schedule of administration, the appropriate apoptotic cell type to be used, as well as the apoptotic signal. We also have analyzed the clinical relevancy of apoptotic-cell-based therapies in experimental models. Additional experimental data are required concerning the treatment of inflammatory diseases (excepted for sepsis) before considering future clinical trials. In contrast, apoptotic cells have been shown to favor engraftment and to reduce acute graft-versus-host disease (GvHD) in different relevant models of transplantation. This has led to the conduct of a phase 1/2a clinical trial to alleviate GvHD. The absence of toxic effects obtained in this trial may support the development of other clinical studies based on this new cell therapy. Stem Cells 2016;34:1464-1473.

Concise Review: Mechanisms Behind Apoptotic Cell-Based Therapies Against Transplant Rejection and Graft versus Host Disease

The main limitations to the success of transplantation are the antigraft response developed by the recipient immune system, and the adverse side effects of chronic immunosuppression. Graft-versus-host disease (GVHD) triggered by donor-derived T lymphocytes against the recipient tissues is another serious obstacle in the field of hematopoietic stem cell transplantation. Several laboratories have tested the possibility of promoting antigen (Ag)-specific tolerance for therapy of graft rejection, GVHD, and autoimmune disorders, by developing methodologies that mimic the mechanisms by which the immune system maintains peripheral tolerance in the steady state. It has been long recognized that the silent clearance of cells undergoing apoptosis exerts potent immune-regulatory effects and provides apoptotic cell-derived Ags to those Ag-presenting cells (APCs) that internalize them, in particular macrophages and dendritic cells. Therefore, in situ-targeting of recipient APCs by systemic administration of leukocytes in early apoptosis and bearing donor Ags represents a relatively simple approach to control the antidonor response against allografts. Here, we review the mechanisms by which apoptotic cells are silently cleared by phagocytes, and how such phenomenon leads to down-regulation of the innate and adaptive immunity. We discuss the evolution of apoptotic cell-based therapies from murine models of organ/tissue transplantation and GVHD, to clinical trials. We make emphasis on potential limitations and areas of concern of apoptotic cell-based therapies, and on how other immune-suppressive therapies used in the clinics or tested experimentally likely also function through the silent clearance of apoptotic cells by the immune system. Stem Cells 2016;34:1142-1150.

Prospects of apoptotic cell-based therapies for transplantation and inflammatory diseases

Apoptotic cell removal or interactions of early-stage apoptotic cells with immune cells are associated with an immunomodulatory microenvironment that can be harnessed to exert therapeutic effects. While the involved immune mechanisms are still being deciphered, apoptotic cell infusion has been tested in different experimental models where inflammation is deregulated. This includes chronic and acute inflammatory disorders such as arthritis, contact hypersensitivity and acute myocardial infarction. Apoptotic cell infusion has also been used in transplantation settings to prevent or treat acute and chronic rejection, as well as to limit acute graft-versus-host disease associated with allogeneic hematopoietic cell transplantation. Here, we review the mechanisms involved in apoptotic cell-induced immunomodulation and data obtained in preclinical models of transplantation and inflammatory diseases.